F-16 wings thinness

Patriot · by **Patriot** » 25 May 2019, 19:13

Basically I was always amazed by how thin F-16 wings are yet how strong they're. It still can carry two 2000 punds bombs + AAMs under each wing and perform some maneuvers / attain some g. How does they did it structually? The loads on the mounts have to be huge. Like for example, F-15 does not carry stuff under its wings despite they're visibly thicker.. maybe theyre less stiff? What is the secret behind Viper's wing load capacity?

johnwill · by **johnwill** » 26 May 2019, 06:19

Load on the wing comes from three sources - air pressure up on the wing, wing weight x g down on the wing, and external store load, mostly down on the wing. By far the largest load is air pressure up, as much as 70,000 lbs. Wing weight x g can be as much as 10,000 lbs. down, and store load can be as much as 40,000 lbs down, all during high g maneuvers and all on each wing.
So you can see in a global sense, carrying stores reduces total wing load. In a local load context, store load attachment bolts do result in large local loads around the bolt holes, but that involves only a small local area.
I read somewhere the F-15 wing has three spars because of "not one pound for air to ground" original design. The F-16 wing contains eleven spars, plus five or six ribs. Although the wings are relatively thin, the skins are thick enough to carry the loads.

Patriot · by **Patriot** » 26 May 2019, 11:34

Thanks John. Make sense.

Do you have any schematics that show the metod the wing is mounted to the fuselage? I guess it got to be so the entire load is spread evenly over the entire tangential area, like wing and fuselage tabs that overlap eachother to make a firm connection? What is the material used? Titanium?

jaws · by **jaws** » 26 May 2019, 14:58

Two large bolts hold wing attach fittings ("finger braces") to the fuselage, 4 on the upper surface and 4 on the lower surface of the wing. Another 19 bolts of various sizes and torque hold the "finger brace" to the wing surface. All the attach hardware is wet sealed as it is installed including the bolts that enter the fuel cells. I don't remember the torque on all the hardware, it's been over 12 years since this former Fuel Systems Technician installed these assemblies.

I'm not sure what the finger braces are machined from, but they are fracture critical, minimal damage is permitted (scratches, nicks, gouges)

Here are a couple images of the attach fittings that secure the wing to the fuselage:

johnwill · by **johnwill** » 26 May 2019, 23:47

In addition to the four wing attach fittings, there are shear ties at the front and rear spar. The attach fittings are aluminum, except titanium was used on the 26 Navy Aggressor airplanes.

jetblast16 · by **jetblast16** » 27 May 2019, 02:29

, except titanium was used on the 26 Navy Aggressor airplanes.

That's awesome. Doubtless to handle serious air loads

johnwill · by **johnwill** » 27 May 2019, 17:16

Original F-16 wing design usage was 45% air to air, 55% air to ground. Since the Navy aggressor usage was entirely air to air, durability analysis showed the need to switch to titanium for the wing attach fittings. That was due to many more very high g maneuvers in air to air. The maximum loads were no higher, just more of them in a lifetime.

Patriot · by **Patriot** » 27 May 2019, 19:07

So if F-16N have strengthened wings compared to any other F-16, why it was withdrawn from service after only 8-10 years?

basher54321 · by **basher54321** » 27 May 2019, 20:33

Discussed before:

….....Some parts of the airplane had an easier time of that (weapon hardpoints, for example), but some parts had a more severe experience of high g events. The N airplanes were delivered with titanium wing attach brackets in place of aluminum brackets on the A/B. The Navy would not pay for a full analysis and durability test of those parts, since they were easily inspected. Bad idea, as the brackets started cracking at a relative young age.

But there is more to the story. USAF uses a structural technology called fracture mechanics to track crack growth and allows airplanes to keep flying with cracks so long as the cracks remain within specified lengths. The Navy, being old fashioned and ultra conservative still used fatigue crack rules which say that any crack is reason for grounding. Because fatigue analysis and test is much more primitive than fracture mechanics, it requires a more conservative approach. So if USAF had those same F-16N airplanes, they would not have grounded them.

viewtopic.php?f=22&t=24886&start=195

johnwill · by **johnwill** » 27 May 2019, 22:06

Thanks, Basher. I have to be careful what I say around here, else someone will dig up old posts to see if I'm just making things up.

saberrider · by **saberrider** » 28 May 2019, 16:31

If the munitions is not too long to add bending moment they could be carried

basher54321 · by **basher54321** » 28 May 2019, 17:02

johnwill wrote:Thanks, Basher. I have to be careful what I say around here, else someone will dig up old posts to see if I'm just making things up.

jacarlsen · by **jacarlsen** » 29 May 2019, 11:21

Hello. I'm wondering where those big wing attach fittings are from. I've fittet alot of WAFs, blokc 10 and 15, pre and post falcon STAR, but I have never seen those big ones.

johnwill · by **johnwill** » 29 May 2019, 22:13

The fittings in the upper photograph are on the upper surface of the wing. Their primary load is compression which is less severe than tension for durability life. The fittings in the lower photo are on the lower surface, thus mostly in tension. That's why they are so much larger. Over the years, both fittings may have been redesigned to account for gross weight increases.

boilermaker · by **boilermaker** » 02 Jun 2019, 02:58

Patriot wrote:So if F-16N have strengthened wings compared to any other F-16, why it was withdrawn from service after only 8-10 years?

Their engine were a unique boosted version with bigger intake. I think they wore out.